1. Field of the Invention
The present invention relates to a heating apparatus of a magnetic (electromagnetic) induction heating system. The present invention also relates to an image forming apparatus comprising the heating apparatus as heat fixing means of an unfixed toner image.
2. Description of the Related Art
For convenience, description is made by taking an example of a fixing apparatus as a heating apparatus for fixing an unfixed toner image on a recording material, which is provided in an image forming apparatus such as a copying machine or a printer.
In such an image forming apparatus, an apparatus of a heat roller system has widely been used as a fixing apparatus for fixing an unfixed toner image of target image information on a surface of a recording material by heating as a permanently fixed image, the unfixed toner image of the target image information having been formed and born on the recording material (transferring material sheet, electrofax sheet, electrostatic recording sheet, OHP sheet, printing sheet, formatting sheet or the like) by proper image forming process means portion such as an electrophotographic process, an electrostatic recording process, or a magnetic recording process based on a transferring system or a direct system.
In recent years, an apparatus of a belt heating system has been put to practical use in order to achieve a quick start and energy conservation and, as a much more efficient fixing apparatus, a fixing apparatus of a magnetic induction heating system has been presented. This fixing apparatus is adapted to apply a magnetic field of magnetic field generating means for generating a magnetic field (high-frequency magnetic field) by receiving supply of power to a magnetic induction heat generating heating member (conductive member) to be fixed or moved, and heat a target material by heat (eddy current loss, or Joule heat) generated by an induced eddy current generated in the heating member. A promising heating member is an induction heating system for causing a belt (film) to generate heat itself.
In the magnetic induction heating system, an inverter circuit is used for a power source (excitation circuit, or high-frequency power source) provided to supply power to an exciting coil of the magnetic field generating means, and there are generally current and voltage resonance systems. The resonance system actively generates a vibration state of a voltage or a current generated during switching, and executes switching to a switching element by choosing a time when a value of the voltage or current, or values of both are lowest, in order to reduce a loss of the switching element for conversion when relatively large power is used. This resonance system also called soft switching is a most effective method when large power is used. Especially, the voltage resonance system has been a mainstream system for an induction heating power source, because it can be provided by a simple configuration.
FIG. 13 shows an inverter circuit of such a voltage resonance system. In the drawing, a reference numeral 271a denotes a switching element such as an IGBT; 271b a resonance coil (exciting coil); 271c a resonance capacitor; and 271d a regenerative diode.
In an operation of the voltage resonance inverter circuit, when the switching element 271a is turned ON to store power in the resonance coil 271b, and then turned OFF, a voltage starts vibrating in an arc of resonance at a cycle decided by constants of the resonance coil 271b and the resonance capacitor 271c. 
A state in the above case is shown in FIG. 14, where a reference numeral 271S denotes a gate switching signal entered to the switching element 271a; 271V a voltage applied to the switching element 271a; and 271I a current flowing to the switching element 271a. 
Power is represented by a product of a voltage and a current. It can be understood from the current 271I and the voltage 2171V that during an operation of the switching element 271a, there are no points of simultaneous application of a voltage and a current, thus generating no power loses.
A temperature of the fixing apparatus of the magnetic induction heating system based on the above voltage resonance system is controlled based on an ON width of the switching element 271a. That is, a power input is increased when an ON width is long, and reduced when an ON width is short.
FIG. 15 shows an operation waveform when a power conversion operation is performed by reducing an ON width of the gate switching signal 271S in order to make output power small. A voltage waveform of the switching element 271a when the output power is made small draws a sine wave resonance-damped with a power supply voltage (level indicated by broken line) terminal-connected to the resonance coil 271b set as a reference. Vibration amplitude of a voltage depends on exciting power stored in the resonance coil 271b, i.e., an ON width of the switching element 271a. This vibration amplitude is small during power saving, and the voltage is not sufficiently lowered from the power supply voltage, making it impossible to obtain zero crossing.
Thus, when the switching element 271a is operated, a current flows always in a voltage applied state, causing a power loss in the switching element 271a. The switching element 271a is driven for switching on an order of xcexcs. If a power loss occurs for each driving, heat generated by the loss becomes very large, creating a possibility that even the switching element 271a itself will be broken.
During continuous printing, when the fixing apparatus is set in a sufficiently heated state, input power must be reduced to about 100 W. However, the input power can only be reduced to about 300 to 400 W if a minimum ON width is set to prevent heat destruction of the switching element 271a. That is, in the conventional fixing apparatus of the electromagnetic induction heating system based on the voltage resonance system, there is inherent a problem that a temperature of the fixing apparatus becomes equal to/higher than a set level during the continuous printing.
An object of the present invention is to enable saved power to be inputted in a heating apparatus of a magnetic induction heating system. Another object is to enable continuous printing to be satisfactorily performed even when a magnetic induction heating system is employed for a heating apparatus (fixing apparatus) of an image forming apparatus.
In order to achieve the foregoing object, in accordance with the present invention, there is provided a heating apparatus of a magnetic induction heating system, comprising: power control for executing variable control from minimum power, not zero, to maximum power in an ON state; and power control for executing control of repetition of the power control between ON and OFF states.
In order to achieve the foregoing object, in accordance with the present invention, there is provided an image forming apparatus comprising: image forming means for forming an bearing an unfixed toner image on a recording material; and fixing means for fixing the unfixed toner image on the recording material by heat. In this case, the fixing means includes power control for executing variable control from minimum power, not zero, to maximum power on an ON state, and power control for executing repetition of the power control between ON and OFF states.
Preferably, when the control for the repetition between the ON and OFF states is carried out, power inputted on the ON state is minimum power to be inputted by the variable control.
According to the present invention, in the heating apparatus of the magnetic induction heating system, by using both ON width control of a switching element of an inverter circuit of a voltage resonance system, and ON-OFF control as a control mode for a power source (excitation circuit) for supplying power to magnetic field generating means (exciting coil), saved power input can be realized in the apparatus. That is, in a normal state, power is controlled by the ON width control of the switching element of the inverter circuit and, when power of a minimum ON width or lower is controlled, control can be executed from small power to large power by fixing the ON width of the switching element to the minimum ON width, and executing ON-OFF repetition control. Thus, continuous printing can be performed satisfactorily even when the magnetic induction system is employed for the heating apparatus of the image forming apparatus.
These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.